Monoolefins such as ethylene and propylene are often employed as feedstocks in preparing value added chemicals and polymers. Monoolefins may be produced by pyrolysis or steam cracking of naphtha, refinery gases, ethane, propane, butane, and the like. Monoolefins produced in this manner usually contain small proportions of acetylenes and diolefins that adversely affect production of subsequent chemicals and polymers. Thus, to form a polymer grade monoolefin, the amount of acetylenes and diolefins recovered in a monoolefin stream is typically reduced. For example, in polymer grade ethylene, it is generally preferred that the acetylene content be less than about 2 ppm, more preferably less than about 1 ppm.
One technique commonly used to reduce the amount of acetylenes and diolefins (i.e., highly unsaturated hydrocarbons) in an unsaturated hydrocarbon stream primarily comprising monoolefins involves selectively hydrogenating the acetylenes and diolefins to monoolefins. This process is selective in that hydrogenation of the monoolefin and the highly unsaturated hydrocarbons to saturated hydrocarbons is minimized. For example, hydrogenation of ethylene to ethane and of acetylene to ethane is minimized. This technique may be performed using catalysts comprising palladium and an inorganic support. However, such catalysts can produce green oil by oligomerizing the acetylenes and diolefins in some circumstances. As used herein, green oil refers to molecules having more than 4 carbon atoms per molecule that undesirably foul the selective hydrogenation catalyst, causing the catalyst to deactivate. The deactivation of the selective hydrogenation catalyst results in the catalyst having a lower activity and selectivity to monoolefins. Green oil production is particularly apparent in backend hydrogenation reactors. As used herein, backend refers to the location of the hydrogenation reactors in an unsaturated hydrocarbon production unit downstream from a deethanizer that receives the higher boiling fraction from a demethanizer as feed. A selective hydrogenation catalyst in such backend reactors requires regular regeneration, and with each regeneration, the selectivity and the life of the catalyst decreases.
Therefore, a need exists for a selective hydrogenation catalyst that undergoes less fouling during the hydrogenation of acetylenes and diolefins, particularly in backend hydrogenation reactors. That is, it is desirable to produce less green oil, thereby extending the life of the catalyst and maintaining catalyst selectivity.